Polyimide processing is a well understood semiconductor art designed to give extra protection to the surface of the silicon chip against scratching, cracking and other types of mechanical damage. Most often, mechanical damage occurs assembly, packaging, or any subsequent handling of the die. As a passivation layer, polyimide also guards against thin film cracking which frequently results from the packaging of very large dies into plastic packages.
Existing polyimide processes are compatible with standard forms of wire bonding technology. Normal defects such as cracks or flakes which arise from the interaction of the polyimide with photoresist, do not normally pose a problem for this standard bonding technology. However, as newer forms of bonding technology come into use--such as tape automated bonding (TAB) which utilizes gold bumps formed on the bonding pads of the semiconductor substrate--the problems associated with prior art polyimide processes have become more troublesome.
For example, one of the main difficulties with prior art polyimide processes is that the profiles (i.e., slopes) of the polyimide at the bonding pad edges are not consistent. Rough edges or films having numerous flakes and other defects are pervasive throughout the prior art. In other cases, pieces of photoresist can sometimes become deposited on the surface of the bonding pads causing spikes of unetched passivation layer to be left behind on the bonding pad itself. Although these problems have not prohibited the use of conventional polyimide processes in conjunction with standard wire bonding techniques, these shortcomings are unacceptable in the newer, more advanced bonding. For instance, TAB compatible gold bump bonding generally requires consistent polyimide edge profiles which are relatively defect-free. Moreover, the slopes of the polyimide openings at the edges of the bonding pads must be formed at such an angle so as to allow good step metal coverage. (During the first stage of the TAB process titanium tungsten (TiW) is sputtered as a barrier metal layer.) If the walls of the polyimide layer are too steep, as in some prior art processes, the barrier metal layer will not adhere properly to the polyimide surface.